1. Field of the Invention
The present invention relates to a high order silane composition and a method of forming a silicon film that can be used in particular with integrated circuits, thin film transistors, photoelectric converters, photoreceptors, and so on. Specifically, the present invention relates to a high order silane composition that in particular enables a high-quality silicon film to be formed easily, and a method of forming an excellent silicon film using the composition.
2. Description of the Related Art
Patterning of silicon thin films (amorphous silicon films, polysilicon films, etc.) used with integrated circuits, thin film transistors, and so on is generally carried out through a process in which a silicon film is formed over the whole surface by a vacuum process such as CVD (chemical vapor deposition), and then unwanted parts are removed by photolithography. However, with this method, there are problems such as a large apparatus being required, the efficiency of usage of the raw material being poor, the raw material being difficult to handle due to being a gas, and a large amount of waste being generated.
Japanese Patent Application Laid-open No. 1-29661 discloses a method in which a gaseous raw material is liquefied and attached on a cooled substrate, and is then reacted with chemically active atomic hydrogen, thus forming a silicon-based thin film. However, there is a problem in that the raw material silane is continuously vaporized and cooled, and hence a complicated apparatus is required, and moreover it is difficult to control the film thickness.
Moreover, Japanese Patent Application Laid-open No. 5-144741 and Japanese Patent Application Laid-open No. 7-267621 disclose methods in which a liquid silane is applied onto a substrate, and then a silicon film is produced through heating or UV irradiation. However, with these methods, a low-molecular-weight material is used, and hence the system is unstable and handling is problematic. Moreover, with these methods, the wettability to the substrate of the solutions used is poor, and hence the application onto the substrate is intrinsically difficult, and moreover the boiling point is low due to the molecular weight being low, and hence during the heating evaporation occurs more quickly than formation of the silicon film, and thus it is extremely difficult to obtain the desired film. That is, how high the molecular weight is (i.e. how good the wettability is, how high the boiling point is, and how good the safety is) of the polysilane (high order silane) used as a material is an important point with regard to film formation.
As a method of resolving the above problem, Japanese Patent Application Laid-open No. 10-321536 suggests a method in which the wettability of a solution of a polysilane is improved by subjecting a mixture of the solution and a catalyst to thermal decomposition or photodecomposition as treatment before the application. With this method, it is necessary to mix a catalyst such as nickel into the solution, and hence there is a drawback that the properties of the silicon film are markedly degraded.
Regarding methods of directly synthesizing silane having a high molecular weight, synthesis procedures and refinement methods are both extremely problematic in general; there have been attempts at a method of directly synthesizing polysilane through thermal polymerization as disclosed in Japanese Patent Application Laid-open No. 11-260729, but at best Si9H20 is obtained and at a low yield, and at such a molecular size, the above-mentioned properties such as wettability are still insufficient.
Moving on, as a method of forming a silicon film containing an n-type or p-type dopant, in general the silicon film is first produced, and then the dopant is introduced therein by ion implantation. In contrast to this, Japanese Patent Application Laid-open No. 2000-31066 discloses a method in which, during a process of forming a silicon film from a polysilane solution as described above, a dopant source is mixed into the material liquid, whereby a doped silicon film is formed. However, with this method as well, there is the fundamental problem for the case of using a low-molecular-weight material that the polysilane solution evaporates during the heating process and hence the amount thereof drops, and accompanying this the dopant source also evaporates, and hence it is difficult to add the dopant effectively.